(1) Field of the Invention
The present invention relates to a high-speed cell search system for CDMA, and a high-speed search circuit for use in a CDMA (Code Division Multiple Access) communication system having larger interfering wave component power than target signal component power, and more particularly to a high-speed cell search system in which S/N (Signal/Noise) is increased by performing coherent integration over plural signals for search, and identification and screening of long codes are performed by using an orthogonal code.
(2) Description of the Related Art
In a CDMA cellular system, a mobile station must establish synchronization of a spreading code sequence before it establishes synchronization of carrier and clocks. It may take long time to establish the code synchronization, and it is a critical problem for CDMA to shorten this time. Specifically, the conventional CDMA cellular system has the following two problems.
A first problem is as follows. That is, the mobile station performs a cell search for detecting a connection cell on the assumption of an estimation method of initial synchronization of spreading codes.
Conventionally, in an inter-base-station synchronization system which is represented by IS-95, the respective base stations synchronously operate in cooperation with one another on the basis of a predetermined offset amount, and thus each base station 1 can perform the cell search work by using only one type of spreading code even when it moves from one base station to another base station, so that the search time can be shortened.
However, in order to use this method, the base stations are required to be synchronized with one another, and in order to satisfy this requirement, a system for generating a reference synchronous signal to establish external synchronization between the base stations is separately needed.
Therefore, an inter-base-station asynchronous system which needs no external system has been required. However, each base station cannot have one common spreading code because the base stations are asynchronous therebetween. Therefore, the respective base stations have different kinds of spreading codes, and thus all kinds of spreading codes must be searched in a process of acquiring and following the initial synchronization, resulting in increase of the search time.
A second problem is as follows. A method of acquiring synchronization and following synchronization on the basis of an coding gain by a Viterbi processor provides high-quality data on convolution-encoded information. However, at the time when the synchronization of the spreading code sequence is established, the Viterbi processor does not operate because it is before data are read in, and thus the detection is performed in the state that there is no coding gain. Since nothing is started (for example, no carrier synchronization is started) unless the synchronization establishment is completed, it is necessary to perform the detection with high quality even when there is no coding gain. That is, if there is some probability that erroneous detection occurs, the repetition of the search work would occur, resulting increase of the search time.
The present invention relates to a search system for use in CDMA having big interference compared with target signal power, and more particularly to a cell search of a long-code mask system for performing high-speed search by using a long-code mask symbol which is subjected to spread spectrum coding with only a short code.
According to the long-code mask system, in a CDMA interbase-station asynchronous cellular system, symbols which are subjected to the spread spectrum coding with a long code are masked in a fixed period to provide symbols (mask symbols) which are subjected to the spread spectrum coding with a short code, and a mobile station implements a high-speed cell search by using the mask symbols (disclosed in xe2x80x9cTechnical Reports RCS 96-74 of Radio Communication System Research of Electronics Information Communication Associates).
According to the long-code mask system which has been open to the public until now, the timing of the long code is detected by utilizing the. mask symbol based on the short code, and then the long code is identified by using a normal sliding correlator. Accordingly, for example, when the number of users is increased to magnify interfering wave power, the probability that the erroneous detection occurs is increased, and the repetition of the search occurs, resulting in increase of the search time. Further, when the number of long codes to be identified is increased, the search time is also increased.
On the other hand, according to the present invention, identification or screening of a long code is enabled by L-time repetition of M mask symbols of the long code (L less than M). Further, coherent integration is performed over M symbols to enhance SNR (Signal-to-Noise Ratio), and specification or screening of the long code is performed at an early stage while reducing the error-occurring probability, thereby implementing the high-speed cell search.
Further, according to the present invention, in a CDMA system which implements the inter-base-station asynchronous system by performing the spread spectrum coding every base station on the basis of codes which are called as xe2x80x9clong codes inherent to the base stationsxe2x80x9d, data comprising orthogonal codes which are constructed by M symbols are added with information for specifying a base station, and the screening or specification of the long code is performed on the basis of the information, so that the high-speed cell search can be implemented.
Further, the present invention is applicable to orthogonal demodulation having I-system and Q-system, and the higher-speed cell search can be performed by performing power addition after the coherent integration is performed.
Still further, the present invention can be implemented with a small scale of circuit by a ring buffer for performing an adding operation with the polarity corresponding to an orthogonal code, so that the present invention is suitably applicable for power save.
According to the present invention, a Barker sequence is used to specify the frame position, and thus data constructed by orthogonal codes can be prevented from being erroneously detected as a frame synchronization signal.
The present invention is applied to a serial search, a parallel search or a serial/parallel mixed search, and a parallel degree, that is, a search detection time can be selected in accordance with an achievable circuit scale, thereby making the design flexible.
According to the present invention, by providing redundancy to data which are constructed by the L-time repetition of the orthogonal codes based on M symbols, data for specifying a long code which is defected due to fading or the like can be restored, thereby reducing the retry frequency of search and shorten the search time as a whole.
Further, according to the present invention, the synchronization of the spreading code sequence is required to be established before the carrier and clock synchronization is established, however, the high-speed search can be performed by performing the coherent integration corresponding to the frequency deviation with a relatively small-scale circuit even in the state that there is a carrier frequency deviation.
A search system for CDMA according to the present invention is characterized in that plural (M) symbols which are subjected to spread frequency coding with a spreading code called as a short code are prepared when synchronization of the spreading code is established before synchronization of carrier is established in a mobile station, data which are obtained by forming an orthogonal code with the polarities of the M symbols are set as a down signal, and when the orthogonal code concerned is detected, coherent integration is performed by a correlator having combinations of the polarities which can be possibly taken by the code over the plural symbols (of M) constituting the code.
Further, in a CDMA system which implements an inter-base-station asynchronous system on the basis of a code called as a long code inherent to a base station, data constructed by the orthogonal code contain information to specify a base station, and specification or screening of a long code is performed on the basis of the information.
Still further, in the present invention, one code of the orthogonal codes is used as a header.
In the present invention, the symbol data of M which are received on the basis of the combination of the polarities determined by the orthogonal codes are added in the in-phase component and the quadrature component, an orthogonal vector which is received on the basis of the square sum thereof or the maximum value of the amplitude component is specified, and the screening or identification of the long code is performed on the basis of the data obtained by the L-time repetition of the above processing.
Further, in the present invention, an M-tap ring buffer which performs addition with the polarities corresponding to the orthogonal codes in each of the I-system and the Q-system is provided, the content of the ring buffer is renewed at a symbol rate and the addition result is used to specify an orthogonal vector.
Still further, in the present invention, a Barker sequence is used to specify the frame position of the M symbols.
In the present invention, the above function is applied to serial search, parallel search or serial/parallel mixed search.
In the present invention, redundancy is provided to codes which are formed by L-time repetition of an orthogonal vector to provide a function of restoring a portion which cannot be detected.
Further, in the present invention, in the coherent integration operation of the M symbols, a sequence which is weighted and polarized on the assumption of the carrier frequency deviation is prepared, and the maximum one is selected on the basis of the correlation thereof, thereby supporting coherent integration having frequency deviation.
Still further, in the present invention, in the coherent integration operation of the M symbols, when a sequence which is weighted and polarized on the assumption of the carrier frequency deviation is prepared, the approximation is performed by using only the polarities.